Development of latent electrostatic images with crystalline toners



United States Patent 3,272,644 DEVELOPMENT OF LATENT ELECTROSTATTC IMAGES WITH CRYSTALLHNE TONERS George R. Nelson, Holiiston, Mass, assignor to Dennison Manufacturing Company, Framingham, Mass., a corporation of Nevada No Drawing. Filed July 31, 1963, Ser. No. 299,084 6 Claims. (Ci. 117--17.5)

This invention relates to electrophotography and more particularly to improvements in the development of electrostatic images and to developers therefor.

The process of electrostatic photography is known and is described for example in US. Patents 2,297,691 and 2,551,582. One such known process comprises (1) inducing an electrostatic charge on a surface coated with a film of photoconductive, insulating material; (2) exposing the charged surface to a light image for the purpose of releasing the charge at points exposed to the light; (3) contacting the surface with finely divided, colored particles, called toner particles, to render the image visible; and (4) fixing the particles by heat either to the charged surface or to another surface to which the particles are transferred. It is known to apply the toner particles either as a dry dusting powder, with or without carrier particles such as glass beads or iron fillings, or as a suspension in a dielectric liquid.

Heretofore useful toners having good coloring power and being capable of being fixed on a surface have usually comprised fine particles of colored polymers or hard waxes. The images are fixed by fusion which causes the polymers and/ or wax to soften and adhere to the surface. These materials are of substantial molecular weight and must have a relatively high softening point to facilitate mechanical comminution and to prevent agglomeration at storage or use temperatures. Relatively high temperatures and undesirably long times are required to fuse such toners since their softening points must be exceeded by a considerable margin to obtain adequate flow. Moreover, such toners do not provide as good tone gradation as desired.

The present invention relates to development of latent electrostatic images in recording elements which comprise finely divided photoconductive particles suspended in an insulating thermoplastic binder on a supporting surface such as paper. It has for its main object the improvement in the development and fixing of image directly on such photoconductive surfaces and to the provision of improved toners therefor. Other and further objects and advantages will be apparent from the following description.

The toners according to this invention are substantially free of high molecular weight resins and waxes and consist essentially of a low molecular weight, crystalline carrier material having dye uniformly dispersed therein, the crystalline material being, when melted, a solvent for substantially all of the dye. The crystalline materials useful herein are organic, that is, carbon containing, including organo-metallic compounds; are chemically nonreactive With dye contained therein; are of low molecular weight not in excess of about 350 with their longest linear carbon chain length not exceeding about fifteen carbon units and preferably less than 10, phenyl groups being considered as one unit; have a melting range within about 1 C. between 140 F. and the charring temperature of the latent image and support, and preferably not greater than about 300 F.; are water-insoluble and, when melted, have a steadily declining viscosity with increasing temperature and viscosity at their lowest melt temperature not substantially exceeding 40 centipoises. Either a single or a mixture of crystalline materials having the above properties can be used, at least one such material present, when melted, a solvating agent for at least one thermoplastic binder resin present in the photoconductive recording element. A substance has sufficient solvating properties for use herein if it will dissolve at least 1% of binder resin, based on its own weight.

The present toners are believed to fix by melting of the crystalline material which flows readily into the photoconductive layer and paper, solvating the photoconductive binder resin and carrying substantially all of the dye into the paper where it is fixed upon cooling to provide a smudge-free print. Since the dye provides the desired color to render the image visible, it should be present in substantial amount, preferably 10 to 60% by weight of the toner. The crystalline material is readily ground and does not agglomerate readily since it is a non-tacky solid. Consequently the toners are readily provided and stored in the requisite finely divided form and rapidly fixed to provide smudge-free prints at relatively low tem peratures.

Toners as defined above are conveniently prepared by melting the crystalline material, dissolving the dye therein, solidifying and comminuting to the desired particle size. The comminuted particles may then be used in known manner either as a dusting powder, with or without mixing with triboelectric charge producing carrier particles such as iron filings or glass beads, or may be suspended in a dielectric liquid to provide a liquid developer.

The following examples are given for the purpose of illustration. All parts are by weight unless otherwise indicated.

Example 1 m-Terphenyl 50 Dye 5 Example 2 Santicizer 1-H 50 Dye 3 10 Example 3 Diphenyl phthalate 50 Dye (Luxol black BN) 10 Example 4 Stearic acid 50 Dye 10 Example 5 Acetoacetanilide 50 Dye 10 Example 6 Biphenyl 50 Dye OJ. No. 24.

Example 7 Santicizer 96 50 Dye 4 10 1 Calco Oil Black B3S04, American Cyanamid Co.

mi 1013 organic sulfonamide, M.P. 86-87 ;C., Monsanto Chemi- In each of the foregoing examples, the binder was melted and the dye dissolved therein. The solution was cooled to solidification and ground at room temperature to pass a 100 mesh screen. The resulting powder was mixed with iron filings in the weight ratio of 40 parts filings for each part powder and then dusted onto a latent electrostatic image. The image comprised paper coated with a layer of photoconductive zinc oxide particles suspended in Pliolite S5D, a high styrene butadiene copolymer of the Goodyear Tire & Rubber Co. The print was fixed by heating for about ten seconds at 100 C. for Examples 1-6, and at 125 C. for Example 7. An excellent smudge-resistant copy with good tone gradation was obtained in each case.

Alternatively, the toners, of Examples 1-7, after solution of dye in the binder and cooling, can be ground or ball milled to the desired size, generally between about 0.1 and 40 microns and more preferably 2-20 microns, and suspended in a known dielectric liquid for application as a liquid toner to a latent electrostatic image. Such suspending liquids should be non-solvents for the toner particles and have a suitable dielectric constant, generally below 3.4 and preferably below 3.0, and a volume resistivity greater than about ohm centimeters. The image is developed by immersion or the like in the toner and is fixed by heat.

Comparisons of the foregoing crystalline toners with commercially available resinous toners indicated that fusion to similar smudge-resistance could be obtained in about one-third of the time required heretofore.

Example 8 below is the currently preferred embodiment.

Example 8 Santicizer 9 60 Dye, Nigrosine SSB 40 In Example 8 above, the Santicizer 9 was melted and the dye dissolved therein. The melt was then cooled and ground under Dry Ice refrigeration to an average particle size of 30 microns. The powder can be further reduced in size by suitable ball milling, if desired. It can be mixed with a known carrier substance to provide the desired triboelectric charge, such as iron filings or glass beads. powder may be mixed with about 40 parts of iron fillings and the mixture dusted onto the latent photoconductive image and fused to the surface at any suitable temperature above about 95 C. For example, 2 seconds at 150 C. has been found satisfactory to produce a smudge-free rint. P More preferably, however, the toner of Example 8 is suspended in a dielectric liquid for liquid development as follows. The ground toner particles are dispersed in the following liquid suspending system to obtain a toner concentrate of solids:

TONER CONCENTRATE Toner particles 70 Dispersant 1 3.5 Aluminum tristearate 0.7

Odorless mineral spirits 2 272.3

For example, a tripolymer prepared from 'a weight mixture of 50 parts of oc-ta-decenyl methacrylate, 10 parts diethyl aminoethyl methacrylate, and 40 parts styrene.

2 Mixture of parafiin hydrocarbons obtained from American Mineral Spirits Company having a distillation range from 352 to 390 F. and a flash point of 125 C.

The above concentrate was ball milled in a porcelain mill with steel balls for 72 hours at room temperature to provide an average particle size of to microns. The toner particles can be suspended in the odorles minerals spirits without dispersion aids, if desired, and will remain sufficiently stable for use for a limited time. However, dispersants are preferred as indicated above to prolong stability. Those given are preferred as more fully described and claimed in copending application Serial No. 343,130, filed February 6, 1964.

The above toner concentrate is diluted for use in the weight ratio of 1.5 part concentrate to 100 parts additional odorless mineral spirits. When contacted with the diluted suspension, as by immersion therein, a latent electrostatic image is visibly developed by the colored toner particles. Fixation for about 4 seconds at 150 C.

For example, one part of the finely ground Example 9 A toner was prepared by combining an aqueous ferric chloride solution with an aqueous solution of sodium dibutyl dithiocarbamate. Iron dibutyl dithiocarbamate was precipitated and dried, and the resulting black solid ground to pass a 200 mesh screen. This powder was dusted onto a charged area of a latent electrostatic image and fused, giving a blackish-brown print. The powder was also mixed with iron filings to give somewhat better definition.

Example 10 Nickel dibutyl dithiocarbamate was prepared according to the preceding example by substituting a nickel chloride solution for the iron chloride. The resulting nickel salt was green in color and had a melting point of 88 C. The green powder was ground to pass 200 mesh and provided prints with good definition. 10% Nigrosine SSB can be dissolved in the green nickel salt to provide a black powder.

Example 11 2 parts of a red dye, C.I. Solvent Red 36, were dissolved in melted iron stearate and the resulting solid cooled and ground to pass 200 mesh to provide a red dusting powder.

Example 12 2 parts of a black dye, Oil Black 51115, made by Allied Chemical Corp., was dissolved in 98 parts of zinc dibutyl dithiocarbamate, the solution cooled and ground to pass 200 mesh.

Example 13 The zinc dibutyl dithiocarbamate of the preceding example can be replaced by calcium or zinc recinoleate.

In a further aspect of the present invention, the dye dispersed in the crystalline material can comprise a leuco dye base which will produce a distinctive color in contact with an electron donating co-reactant, the co-reactant being incorporated in the photoconductive coating containing the latent electrostatic image to be developed. Such dye bases are usually colorless and hence provide a cleaner toner composition. Suitable leuco dye bases are disclosed in US. Patents 2,695,245; Re. 23,024, 2,981,733 and 2,983,756; the latter two patents disclosing materials known as leuco auromines. Crystal violet lactone is disclosed in Re. 23,024 is preferred. Suitable c-o-reactant electron donors are known and are disclosed in the foregoing patents. Examples include atta-pulgite, bentonite, Santicizer 9, phthalic anhydride, and Hyp-alon 30, a chloros'ulfonated polyethylene sold by the duPont Company. Other suitable electron donor co-reactants will be apparent to persons skilled in the art.

Suflicient donor co-reactant should be incorporated in the photoconductive coating to provide good color development while excess should be voided since it can adversely affect the electrophotographic properties of the coating. Between about 4 and 25% by Weight co-reactant, based on the weight of photoconductive material in the coating is preferred.

The crystalline material used with the leuco dye base should be a material which is non-reactive with the leuco dyes. Suitable binders include m-terphenyl, o-terphenyl, myristamide, biphenyl, and naphthalene.

Examples 14-22 below illustrate suitable leuco toner systems.

A styrene butadiene copolymer of the Goodyear Tire & Rubber Co.

2 A modified styrene copolymer of the Pennsylvania Industrial Chemical Co.

i'hn aromatic resin of the Pennsylvania Industrial Chemi- Cft l O.

The resins above were dissolved in the solvents and the photoconductive zinc oxide and the attapulgite dispersed therein. The suspension was then ball milled in a :mill with steel balls for 16 hours at room temperature. Fifteen pounds of this suspension per 20 x 25500 ream were then coated on a 60 pound clay-coated paper and dried.

B. Toner (a) Dusting powder:

parts crystal violet lactone were dissolved in 90 parts melted m-terphenyl. The solution was cooled, ground to pass a 200 mesh screen and mixed with iron filings in the ratio of 2.0 parts per 100 parts by weight iron filings.

(b) Liquid toner:

10 parts crystal violet lactone were dissolved in 90 parts melted myrist'amide and cooled. 20 parts thereof were suspended in 30 parts Odorless Mineral Spirits and 2.0 parts of the dispersant of Example 8 above and ball milled. 2 grams of this ground suspension was then diluted with 100 cc. additional of Odorless Mineral Spirits to form the developer.

Application of either toners (a) or (b) to a charged image of coating A produced, on fusion of the toner at 150 C., a deep blue visible reproduction of the latent image.

Example Same as Example 14A except that 175 parts zinc oxide and 58.6 parts attaplugite were employed.

Example 16 Same as Example 14A excepting that Hypalon Was substituted for the attaplugite.

Example 17 Same as Example 14A excepting that Santicizer 9 was substituted for the attapulgite.

Example 18 A. Photosensitive paper coating:

Zinc oxide 117 Toluene 70 Acetone 15 Pliolite S-SD 20.25 Piccoflex 120 13.5 Piccocizer 40 1.65 Rose Bengal .0084 Bentonite clay 5 After ball milling as in Example 15, the above mixture was coated, 9 pounds per 20 x 500 ream, and dried at 100 C.

B. Toner (a) Dusting powder:

0.5 to 4 parts of a leuco Auromine, prepared according to US. Patents 2,981,733 and 2,983,756 was dissolved in 10 parts melted m-terphenyl, cooled and ground to pass a 100 mesh screen. 10 parts iron filings were added for each part ground dye-binder mixture.

(b) Liquid tone:

4.5 parts of the ground dye binder mixture of 18-B (a) above were suspended in 10 parts odorless mineral spirits and 0.3 part dispersant. This suspension was then ball milled, using steel balls, for 40 hours. 0.5 part of this concentrate were then dispersed in 75 additional parts odorless mineral spirits and 25 parts of silicone oil, DC- 200 of the Dow Corning Chemical Company having a 1.5 centistoke viscosity.

The paper according to 18A was stored in the dark for 24 hours, charged with a negative corona at 6000 volts, and exposed to light through a transparent negative to produce a latent electrostatic image. When the dusting toner of 18B (a) was brushed across the negative, the toner adhered to the charge areas and heating to C. produced a permanent blue print. Immersion of the latent image in toner 19B (h) followed by drying at 125 C. likewise produced a permanent blue image.

Example 19 Same as Example 18A excepting that 25 parts bentonite was employed.

Example 20 Same as Example 18A excepting that 10 parts Santicizer 9 was substituted for the bentonite.

Example 21 Same as Example 18A excepting that 10 parts phthalic anhydride was substituted for the bentonite.

Example 22 Same as Example 18A excepting that 9.0 parts Hypalon 30 was substituted for the bentonite.

The toner examples given herein produce positive prints, that is, the color is deposited over negatively charged image areas in direct proportion to the charge.

It should be understood that the foregoing description is for the purpose of illustration and that the invention includes all modifications falling within the scope of the appended claims.

I claim:

1. The method of developing a latent electrostatic image on a coating of photoconductive particles suspended in a thermoplastic resin on a support, said method comprising contacting said image with finely divided solid toner particles consisting essentially of dye dispersed in a water insoluble crystalline organic material non-reactive with said dye, said dye constituting from 10 to 60 percent by weight of said toner particles, said material having a melting range within about 1 C. between about F. and the charring temperature of said resin and support, said material, when melted, being a solvent for substantially all of said dye and a solvating agent for said thermoplastic resin, said melted material having a steadily declining viscosity with the temperature and having at its lowest melt temperature a viscosity not exceeding about 40 centipoises, said material having its longest linear carbon chain not exceeding 15 carbon units and a molecular weight not in excess of about 350, and fusing the toner particles adhering to said image to cause the dye to be absorbed by said coating.

2. The method according to claim 1 wherein said toner particles are suspended in an insulating dielectric liquid which is a nonasolvent for the toner.

3. The method according to claim 1 wherein said material has a melting range between about 140 F. and 300 F.

4. The method of developing a latent electrostatic image on a coating of photoconductive particles suspended in a thermoplastic resin on a support which includes the steps of incorporating an electron donor material co-reactive with a colorless leuco dye base in the photoconductive coating, providing an electrostatic latent image in said coating, contacting said image with finely divided solid toner particles consisting essentially of a leuco dye base dispersed in a crystalline material which is non-reactive With said dye base, said material having a melting range within about 1 C. between about 140 F. and the scorching temperature of said coating and support, said binder, when melted, being a solvent for substantially all of said dye and a solv-ating agent for the thermoplastic resin of said coating, said material having a steadily declining viscosity with temperature and a viscosity not substantially exceeding 40 centipoises when melted, said material having a molecular weight not substantially exceeding 350 and its longest linear carbon chain not exceeding 15 carbon units, said leuco dye base constituting from 10 to 60 percent by weight of said toner particles, and fusing the particles adhering to said image to cause the dye to be absorbed by the coating and produce color by reaction with the co-reactant therein.

5. The method according to claim 4 wherein said electron donor co-reactant is incorporated in said coating in an amount between about 4 and 25% of the weight of photoconductive material contained in said coating.

6. The method according to claim 4 wherein said leuco dye is crystal violet lactone.

References Cited by the Examiner UNITED STATES PATENTS Schladebach et a1 864 Schlack 86 Bainbridge et a1 8-6 Straughan 117--37 Solar 25262.1 Green et al. 117-17.5 Greig 117-37 Greig 117-37 Claus 117-175 Schmiedel et al. 25262.1 Tomanek et al l17 17.5

Assistant Examiners. 

1. THE METHOD OF DEVELOPING A LATENT ELECTROSTATIC IMAGE ON A COATING OF PHOTOCONDUCTIVE PARTICLES SUSPENDED IN A THEREMOPLASTIC RESIN ON A SUPPORT, SAID METHOD COMPRISING CONTACTING SAID IMAGE WITH FINELY DIVIDED SOLID TONER PARTICLES CONSISTING ESSENTIALLY OF DYE DISPERSED IN A WATER INSOLUBLE CRYSTALLINE ORGANIC MATERIAL NON-REACTIVE WITH SAID DYE, SAID DYE CONSTITUTING FROM 10 TO 60 PERCENT BY WEIGHT OF SAID TONER PARTICLES, SAID MATERIAL HAVING A MELTING RANGE WITHIN ABOUT 1*C. BETWEEN ABOUT 140*F. AND THE CHARRING TEMPERATURE OF SAID RESIN AND SUPPORT, SAID MATERIAL, WHEN MELTED, BEING A SOLVENT FOR SUBSTANTIALLY ALL OF SAID DYE AND A SOLVATING AGENT FOR SAID THERMOPLASTIC RESIN, SAID MELTED MATERIAL HAVING A STEADILY DECLINING VISCOSITY WITH THE TEMPERATURE AND HAVING AT ITS LOWEST MELT TEMPERATURE A VISCOSITY NOT EXCEEDING ABOUT 40 CENTIPOISES, SAID MATERIAL HAVING ITS LONGEST LINEAR CARBON CHAIN NOT EXCEEDING 15 CARBON UNITS AND A MOLECULAR WEIGHT NOT IN EXCESS OF ABOUT 350, AND FUSING THE TONER PARTICLES ADHERING TO SAID IMAGE TO CAUSE THE DYE TO BE ABSORBED BY SAID COATING. 